AbstractTo investigate if theSalmonella/microsome assay could reliably screen complex petroleum samples for their carcinogenic potential, two high boiling (700–1070 °F) petroleum distillates with known activity in a dermal carcinogenesis bioassay were fully characterized with respect to their hydrocarbon composition and polynuclear aromatic hydrocarbon (PNA) content and assayed for mutagenic activity. Mutagenicity assays were also carried out on the aromatic hydrocarbon aggregates separated from these oils by adsorption chromatography. The composition of the distillates differed substantially, and reflected the fact that they were derived from crude oils that were extremely divergent in hydrocarbon character. Both the distillate and aromatic samples consistently induced a very slight increase in revertant TA98 and TA100 colonies; however, an increase of 2–4‐fold over background was observed when the S‐9 concentration was increased 5–10 times that of the standard assay. The maximal response was less than that expected from the samples' known PNA content and observed potency in the dermal carcinogenesis bioassay. In theSalmonella/microsome assay, all samples inhibited the mutagenic activity of added benzoapyrene. Discordance between the magnitude of the samples' mutagenic activity and their known PNA content may be related to direct or indirect inhibition of sample PNAs by other components of the complex petroleum fractions. Observed inhibitory effects support the use of elevated S‐9 concentration in thein vitroassays assessing the carcinogenic potential of petroleum‐der
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